Angular-precision enhancement in free-space micromachined optical switches

ABSTRACT

Integrated mechanical angular alignment-enhancement structures for incorporation into free-space micromachined optical switches are capable of achieving better than 0.1° micro-mirror angular precision and repeatability with the optical switches. The structures open the path to high port-count crossconnects with sufficiently low loss for deployment in practical optical-communications networks.

[0001] This nonprovisional application claims the benefit of U.S.Provisional Application No. 60/137,838, filed Jun. 7, 1999.

BACKGROUND OF THE INVENTION

[0002] 1. Field of Invention

[0003] This invention relates to free-space micro-electro-mechanicalsystem (MEMS) optical switches. In particular, it relates to mechanicalangular alignment enhancement structures associated with free-rotatinghinged micro-mirror switching elements.

[0004] 2. Description of Related Art

[0005] As the demand for data-networking capacity has grown, managingoptical networks at the coarse granularity of the wavelength level(OC-48 and beyond) has become increasingly critical. Opticalcrossconnects (OXCs) with high port-count—on the order of 1,000—areemerging as the chief candidates for achieving this end. This vision,however, imposes stringent requirements on the OXCs, chiefly in theareas of port-count and loss budget, that far outstrip availabletechnology. Recently, free-space optical MEMS (micro-electro-mechanicalsystems) have begun to show promise for this application, due largely tothe combined merits of free-space optics and integrated photonics.

[0006] Free-space MEMS optical switches aiming at large-scale switchfabrics have been demonstrated using various approaches. See, forexample, (1) H. Toshiyoshi and H. Fujita, “Electrostatic Micro TorsionMirrors for an Optical Switch Matrix,” J. MicroelectromechanicalSystems, vol. 5, no. 4, pp. 231-237, 1996, (2) L. Y. Lin, E. L.Goldstein, and R. W. Tkach, “Free-space Micromachined Optical Switchesfor Optical Networking,” IEEE J. Selected Topics in Quantum Electronics:Special Issue on Microoptoelectromechanical Systems (MOEMS), vol. 5, no.1, pp. 4-9, 1999, (3) R. T. Chen, H. Nguyen, and M. C. Wu, “A LowVoltage Micromachined Optical Switch by Stress-induced Bending,” in 12thIEEE International Conference on Micro Electro Mechanical Systems,Orlando, Fla., Jan. 17-21, 1999, and (4) B. Behin, K. Y. Lau, and R. S.Muller, “Magnetically Actuated Micromirrors for Fiber-optic Switching,”in Solid-State Sensor and Actuator Workshop, Hilton Head Island, S.C.,Jun. 8-11, 1998, each incorporated by reference herein in theirentireties.

[0007] This work has rapidly revealed the intrinsically good opticalquality of free-space interconnects, particularly in the areas ofcrosstalk, polarization- and wavelength-independence, and bit-ratetransparency. The demonstrated switching times are also well-suited forthe applications of OXCs in core-transport networks.

[0008] However, the issue of tight optical-alignment tolerances infree-space optics remains to be solved.

SUMMARY OF THE INVENTION

[0009] It is therefore an object of the invention to design micro-mirrorswitching elements with enhanced angular precision and repeatability soas to improve the coupling efficiency of the switch and reduce losses inoptical signal intensity as a result of the switching.

[0010] This and other objects are achieved by the present invention thatincludes integrated mechanical angular alignment-enhancement structuresthat are capable of achieving better than 0.1° micro-mirror angularprecision and repeatability, thus opening the path to high port-countoptical crossconnects that live within cross-office optical-lossbudgets.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 is a SEM (scanning electron microscope) photograph of thefree-rotating hinged micro-mirror of the invention.

[0012]FIG. 2 is a SEM photograph detail of the pushrod of FIG. 1.

[0013]FIG. 3 is a schematic diagram of a microactuated free-rotatingswitch mirror.

[0014]FIG. 4 is a SEM photograph of the micro-hinge and the push-barstructure for hinge-position enhancement.

[0015]FIG. 5 is a graph of the measured horizontal angular variationwith and without the push-bars.

[0016]FIG. 6 is a diagram of a conventional scratch drive actuator (SDA)design, where the stop position of the SDA is not clearly defined.

[0017]FIG. 7 is a diagram of the improved SDA design of the invention,where the stop position of the SDA is clearly defined when thefront-wall of the bushing hits the stop block.

[0018]FIG. 8 is a graph of the measured vertical angular variation forboth (1) the improved SDA only and (2) the improved SDA with the HJE(hinge-joint enhancement).

[0019]FIG. 9 is a diagram layout of a conventional hinge joint.

[0020]FIG. 10 is a diagram layout of the improved hinge joint of theinvention. The hinge-pin position is restrained in the improved design.

[0021]FIG. 11 is a diagram of the basic rotational components of themicro-mirror.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0022] In free-space MEMS optical switches, free-rotating hingedmicro-mirrors are utilized as the switching elements.

[0023] Optical switches function to switch an optical signal from aninput port, for example an input fiber, to an output port, for examplean output fiber. The optical switches are located within an open, freespace. The size of the matrix of input and output ports is N×M, with Nand M being any integer greater than 1. An optical micro-mirrorswitching element is positioned, for example, at a 45° angle to thedirection of an incoming optical signal from an input port, and at theintersection of each path from an input port to an output port. Incomingoptical signals may be directed to the desired output port through useof the micro-mirror optical switches.

[0024] When an incoming optical signal is not to be redirected(switched) by a particular micro-mirror, the micro-mirror remains in itsrest position, which is horizontal to the substrate upon which themicro-mirror is mounted, or is at least out of the plane of travel ofthe optical signal. However, if an optical signal is to be switched bythe micro-mirror, the micro-mirror is raised to its reflecting position,which is a predetermined position and preferably is, for example, asclose to perpendicular, i.e., 90°, from the substrate as possible. Inthis position, an incoming optical signal from an input port can beredirected to the desired output port.

[0025] The micro-mirrors of the invention may be made of anyconventional material. For example, the micro-mirrors may bepolysilicon, preferably made by surface micromachining, optionallycoated with a highly reflective metal such as gold or Cr/Au, for exampleas in H. Toshiyoshi and H. Fujita, supra.

[0026] The use of free-rotating hinged micro-mirrors eliminates thepotential problem of long-term material memory and fatigue, both ofwhich are significant issues in OXCs used as facilities switches, withconfiguration-holding-times that may be measured in years.

[0027] One pays for this advantage of free-rotating hinge structures,however, in the form of decreased angular precision. The presentinvention, however, addresses this issue through the inclusion ofenhanced integrated-mechanical designs for the free-rotating hinges andmicro-actuators that achieve a better than 0.1° angular precision. Thedemonstrated angular precision shows the promise of this technology inachieving practical loss budgets in the presence of realisticfabrication constraints. Since free-space MEMS-based opticalcrossconnects employing single-mode fibers require angular precisionbetter than 0.1°, mechanisms for ensuring precise angular control of theconstituent micro-mirrors are important.

[0028] The free-rotating micro-mirrors 20 of the invention, for exampleas illustrated in FIGS. 1, 2, 3 and 11, comprise a mirror (or mirrorframe) connected to the substrate 15 by free-rotating micro-hinges 30.The hinges 30 include one or more hinge pins 22 and one or more hingestaples 24. Pushrods 26 are connected at one end to the mirror (mirrorframe) and at the opposite end to the translation stage 18 with hingejoints 35.

[0029] The micro-mirror, hinge and staple may be formed by anyconventional process. The hinged micro-mirrors may be formed, forexample, by the well-known MUMPs™ (the multi-user MEMS) processdescribed in, for example, L. Y. Lin, E. L. Goldstein and R. W. Tkach,“Free-Space Micromachined Optical Switches for Optical Networking”, IEEEJournal of Selected Topics in Quantum Electronics, vol. 5, no. 1, pp.4-9, January/February 1999 and K. S. J. Pister, M. W. Judy, S. R.Burgett, and R. S. Fearing, “Microfabricated Hinges,” Sensors andActuators A, vol. 33, pp. 249-256, 1992, both incorporated herein byreference in their entireties. In MUMPs™, a polysilicon is used as thestructural material, a deposited oxide (PSG) is used for the sacrificiallayers, and silicon nitride is used as an electrical isolation layerbetween the silicon substrate and the polysilicon layers. Thepolysilicon layers are referred to as poly-0, poly-1 and poly-2.

[0030] Currently, scratch-drive actuators (SDAs) 45 are employed to movethe translation stage. SDAs are conventional, and thus an extensivediscussion of the function of the SDAs is not necessary herein. For adiscussion of the formation and function of SDAs see, for example, T.Akiyama and H. Fujita, “A Quantitative Analysis of Scratch DriveActuator Using Buckling Motion,” in IEEE Workshop on Micro ElectroMechanical Systems, Amsterdam, the Netherlands, Jan. 29-Feb. 2, 1995,incorporated herein by reference in its entirety.

[0031] For purposes of explaining the functioning of the hingedmicro-mirrors of the present invention, it is sufficient to note thatthrough application of an appropriate voltage to the SDAs, the SDAs canbe deformed or moved to a certain extent, which deformation or movementis used to move the translation stage a translation distancecorresponding to the extent of deformation. Movement of the translationstage in turn causes the pushrods to act upon the mirror and rotate itto a predetermined position or angle from the substrate, typically the90° position discussed above.

[0032] Imprecision in the angular alignment of the micro-mirror isintroduced as a result of both the inherent clearance between the hingepins and the hinge staples of the mechanism (which may be on the orderof, for example, 0.75 μm) and uncertainty in the translation distance ofthe micro-actuators. However, we have developed in this inventioneffective mechanical angular alignment-enhancement structures thatregister micro-mirror angular position independently in the twoorthogonal axes, Θ_(x) and θ_(y) (defined in FIG. 1).

[0033] It should be noted here that the thickness, clearance, and otherlength unit measurements described herein are for the purposes ofillustration. The values may vary, depending on the process used tomanufacture the free-space micromachined optical switch chips. That is,the thickness of the layers in the MUMPs process may vary, which in turnvaries the illustrative example values set forth herein.

[0034] The angular uncertainty in θ_(y) is caused mainly by clearance inthe fixed microhinges. If, for example, the mirror frame has a width of,e.g., 450 μm and the inherent hinge-pin clearance is, e.g., 0.75 μm, themaximum mirror-angular variation would be tan⁻¹((0.75×2)/450)=0.19°,which degree of angular imprecision will cause substantial coupling loss(i.e., loss in optical signal transmission intensity after theswitching).

[0035] To register the hinge-pin position with precision surpassing the0.19° just mentioned, polysilicon push-bars 40 (FIGS. 4 and 11) areintegrated with the translation stage according to a first embodiment ofthe present invention, for example as shown in FIG. 4. As thetranslation stage moves forward, the push-bars on both sides of themirror frame also move forward, and eventually push the hinge pinsagainst the inner front-side 33 of the inside of the hinge staples, thuseliminating the hinge-pin clearance and uncertainty in hinge-pinposition. This then allows one to attain angular precision that islimited only by photolithography precision (i.e., the formation processprecision), independent of hinge-pin clearances.

[0036] The push-bars may have any suitable configuration, so long as thepush-bars are able to contact the mirror frame and/or the hinge pin ofthe micro-mirror and press it against a forward inner wall of one ormore of the hinge staples, while at the same time having a height lessthan the height of the overall hinge joint so as not to interfere withthe rotation of the micro-mirror.

[0037] The push-bars may be formed by any suitable process. Preferably,the push-bars are integrally formed upon the translation stage bysurface micromachining of polysilicon, i.e., they are preferably formedduring the same MUMPs process discussed above.

[0038] To measure the improvement in angular location repeatability whenthe push-bars are used, a micro-mirror fabricated with integratedpush-bars is actuated repeatedly, and the mirror angle of the actuatedmicro-mirror is obtained by measuring the position of the reflectedoptical beam on a distant screen. The same measurement is then repeatedfor a micro-mirror without the push-bars. FIG. 5 shows the experimentalresults. With the push-bars, the maximum θ_(y) variation is reduced from0.056° to 0.034°. The results without the push-bars are also much morewidely varied.

[0039] Use of the push-bars as an integrated mechanical angularalignment-enhancement structure thus significantly improves upon theangular precision of the micro-mirrors.

[0040] The vertical angular variation (θ_(x)) appears to depend mainlyon three variables: (1) the clearance in the fixed micro-hinges, (2) theclearance in the hinge-joints at the pushrod endpoints and (3) themoving distance of the translation stage. The first source of variationis squeezed out by the push-bar structures as described above. However,even with this substantial improvement through the use of push-bars,without further improvement in the other two variables, a verticalangular variation greater than 1° has still been observed, due toangular uncertainties in the orthogonal direction. Two additionalmechanical angular alignment-enhancement structures, separatelydescribed below, may thus also preferably be employed in the invention.

[0041] With the scratch-drive actuators (SDAs), the moving distance ofthe translation stage effected by the SDAs is mainly determined by thetraveling distance of the SDAs. In conventional SDA design wherefabrication tolerance is considered, the bushing of the SDA is locatedat a position approximately 2 μm from the advancing edge of the SDA asshown in FIG. 6. The SDA travels until it hits the stop block 65 on thesubstrate.

[0042] The most reliable way to make a stop block with vertical sidewalls and without affecting the flatness of the underlying siliconnitride layer is by utilizing the poly-0 layer in the MUMPs processdescribed above. This layer may have a thickness of, for example, ˜0.5μm. The bushing of the SDA is defined by the poly-2 layer atop thepoly1-poly-2-via. The bushing may have a nominal height of, for example,0.75 μm. Since these two heights (i.e., the height of the poly-0 layerand the height of the bushing) are comparable, and the SDA height variesunder actuation (as explained in T. Akiyama and H. Fujita, supra), theposition at which the SDA comes to rest against the stop block 65varies. Experimentally, various positions between A and B in FIG. 6 havebeen observed during actuations, resulting in angular uncertainties onthe order of, and greater than, 1°.

[0043] In order to eliminate this uncertainty, use is made herein of aspatially extended poly-1-poly-2-via pattern, as shown in FIG. 7. TheSDA forward edge is thus vertically defined by ion etching, for exampleas in the MUMPs process discussed above. As can be seen from FIG. 7, thepoly-1-poly-2-via is extended. Through this design, a true “L”-shapedSDA is formed, in which the bushing 60 of the SDA 45 abuts, or is at, anend outer edge of the SDA. This acts to always stop the SDA when thefront of its bushing contacts the stop block 65, adding certainty to thestop point unlike the SDA design of FIG. 6.

[0044] To measure the resulting improvement in angular precision withthe L-shaped SDA, a micro-mirror with the L-shaped SDA structure isactuated repeatedly. The angular variation is plotted as circular datapoints in FIG. 8. The angular precision is seen to improve from 1° withthe SDA of FIG. 6 to 0.15° with the SDA of FIG. 7 of the invention.

[0045] Another uncertainty in controlling vertical angular precisionarises from clearance in the free-rotating hinge joints. In a priorhinge joint, the hinge pin has a uniform width of, for example, 2 μm.During actuation, the poly-2 pushrods attached to the hinge pin areallowed to rotate and slide freely in a 6-μm slot on the poly-1translation plate. See FIGS. 1, 2, 3 and 4. This yields uncertainty inthe position of the hinge pin, and therefore in the angle of the pushrodand the micro-mirror. To address this uncertainty, a further mechanicalalignment-enhancement structure has been developed in this invention. Inparticular, an improved free-rotating hinge joint is designed and foundto minimize the effect of the clearance in the free-rotating hingejoints.

[0046]FIGS. 9 and 10 illustrate the layout for the original and theimproved hinge joints. The translation stage is connected to the poly-1layer, which has a thickness of, for example, 2 μm. The hinge staple ismade from, for example, the poly-2 layer. The clearance between thepoly-2 hinge staple and the substrate (the area where the hinge pins go)may be, for example, 2.75 μm. This is measured from the height of thesubstrate to the interior edge of the hinge staple.

[0047] In the design of this invention, the part of the hinge pin thatis not covered by the poly-2 hinge staples (portion 100 in FIG. 3) isenlarged to a width greater than the width of the hinge pin within thehinge staples by, for example, 0.3 μm, preferably 0.5 μm, or more. Thus,if the hinge pins within the hinge staples have a width of 2 μm, thehinge pins outside of the hinge staples should have a width of at least2.3 μm, and most preferably have a width of about 2.5 μm. The hinge pinwithin the hinge staple retains a smaller width to preserve itsflexibility.

[0048] As discussed above, the mirror switch is formed in the MUMPsprocess. The hinge pins are thus formed photolithographically. In orderto adjust the width of the hinge pins, it is simply a matter of alteringthe geometry of the photolithographic pattern of the hinge pin layout asviewed from above. This is illustrated in FIGS. 9 (conventional layout)and 10 (improved layout of the present invention).

[0049] As the pushrod is rotated with this hinge pin arrangement, thehinge pin starts to touch contact the hinge staple when the mirror angleapproaches the predetermined position, for example 90° (pushrod angle70°), because of the greater width of the hinge pin at portions outsideof the hinge staples. The pressure from the hinge staple presses thehinge pin down and backward, thus preventing it from sliding and therebystill further improving the angular variation.

[0050] With this hinge-joint enhancement (HJE) structure in addition tothe improved SDA described earlier, the maximum angular variation isfurther reduced to 0.074°, as shown in FIG. 8.

[0051] As has been discussed above, the maximum improvement in angularprecision is achieved when all three of the integrated mechanicalangular alignment-enhancement structures described herein are utilizedtogether. However, it is also an aspect of this invention to use any ofthe integrated mechanical alignment-enhancement structuresindependently, or any combination of two of the mechanical angularalignment-enhancement structures together, in any particular applicationto derive the benefits offered by the integrated mechanicalalignment-enhancement structures selected.

[0052] We have thus designed and demonstrated integrated mechanicalstructures that substantially enhance the angular repeatability of aMEMS crossconnect's micro-mirrors beyond limits that were observed insimple, first-generation devices. Measurements show that angularprecision better than 0.1° can be achieved. The results point towardswhat appears to be a practical route to optical crossconnects that offerboth the port-count and the loss budgets needed in emerginghigh-capacity core-transport networks.

What is claimed is:
 1. An optical switching element comprising: amicro-mirror attached to a substrate through one or more hinge jointscomprised of a hinge pin and a hinge staple, the hinge staple beingattached to the substrate, one or more pushrods associated with themicro-mirror, and capable of rotating the micro-mirror upon movement ofa translation stage, and at least one mechanical angularalignment-enhancement structure.
 2. The optical switching element ofclaim 1 , wherein the micro-mirror is mounted within a frame integralwith the hinge joints.
 3. The optical switching element of claim 1 ,wherein the pushrods are attached at one end to the micro-mirror by ahinge joint and are attached at an opposite end to the translation stageby a hinge joint.
 4. The optical switching element of claim 1 , whereinthe at least one mechanical angular alignment-enhancement structure isselected from the group consisting of one or more push-bars upon thesubstrate, one or more scratch drive actuators having a bushing abuttingan end edge of the actuator, and one or more hinge pins having a widthin portions of the hinge pin outside of the hinge staples greater than awidth of portions of the hinge pin inside of the hinge staples.
 5. Theoptical switching element of claim 1 , wherein the at least onemechanical angular alignment-enhancement structure is one or morepush-bars upon the substrate.
 6. The optical switching element of claim5 , wherein the one or more push-bars are movable upon movement of thetranslation stage.
 7. The optical switching element of claim 5 , whereinthe one or more push-bars move to press one or more hinge pins of themicro-mirror against a forward inner wall of one or more of the hingestaples.
 8. The optical switching element of claim 1 , wherein the atleast one mechanical angular alignment-enhancement structure is one ormore scratch drive actuators having a bushing at an end edge of theactuator.
 9. The optical switching element of claim 1 , wherein the atleast one mechanical angular alignment-enhancement structure is one ormore hinge pins having a width in portions of the hinge pin outside ofthe hinge staples greater than a width of portions of the hinge pininside of the hinge staples.
 10. The optical switching element of claim9 , wherein the portions of the hinge pin outside of the hinge stapleshave a width greater than the width of portions of the hinge pin insideof the hinge staples by 0.3 μm or more.
 11. The optical switchingelement of claim 10 , wherein the portions of the hinge pin outside ofthe hinge staples have a width greater than the width of portions of thehinge pin inside of the hinge staples by 0.5 μm or more.
 12. A method ofachieving angular alignment precision of at least 0.1° with amicro-mirror of an optical switching element at a predetermined anglefrom a substrate upon which the micro-mirror is mounted when rotatingthe micro-mirror to the predetermined angle, the optical switchingelement comprising the micro-mirror attached to a substrate through oneor more hinge joints comprised of a hinge pin and a hinge staple, thehinge staple being attached to the substrate, one or more pushrodsassociated with the micro-mirror, and capable of rotating themicro-mirror upon movement of a translation stage, and at least onemechanical angular alignment-enhancement structure, the methodcomprising moving the translation stage with one or more scratch driveactuators to move the pushrods and rotate the micro-mirror via the oneor more hinge joints, and positioning the micro-mirror at thepredetermined angle within 0.1° precision, the precision achieved viathe at least one mechanical alignment-enhancement structure.
 13. Themethod according to claim 12 , wherein the at least one mechanicalangular alignment-enhancement structure is selected from the groupconsisting of one or more push-bars upon the substrate, one or morescratch drive actuators having a bushing at an end edge of the actuator,and one or more hinge pins having a width in portions of the hinge pinoutside of the hinge staples greater than a width of portions of thehinge pin inside of the hinge staples.
 14. The method according to claim12 , wherein the at least one mechanical angular alignment-enhancementstructure is one or more push-bars upon the substrate, the one or morepush-bars moving upon movement of the translation stage to press one ormore hinge pins of the hinge joints against a forward inner wall of oneor more of the hinge staples.
 15. The method according to claim 12 ,wherein the at least one mechanical angular alignment-enhancementstructure is one or more scratch drive actuators having a bushing at theend edge of the actuator, the scratch drive actuators effecting precisemovement of the translation stage to achieve the precision in thepositioning of the micro-mirror.
 16. The method according to claim 12 ,wherein the at least one mechanical angular alignment-enhancementstructure is one or more hinge pins having a width in portions of thehinge pin outside of the hinge staples greater than a width of portionsof the hinge pin inside of the hinge staples by 0.5 μm or more.
 17. Amethod of rotating a micro-mirror of an optical switching element to apredetermined position from a substrate upon which the micro-mirror ismounted, the optical switching element comprising the micro-mirrorattached to a substrate through one or more hinge joints comprised of ahinge pin and a hinge staple, the hinge staple being attached to thesubstrate, one or more pushrods associated with the micro-mirror, andcapable of rotating the micro-mirror upon movement of a translationstage, and at least one mechanical angular alignment-enhancementstructure, the method comprising moving the translation stage with oneor more scratch drive actuators to move the pushrods and rotate themicro-mirror via the one or more hinge joints, and positioning themicro-mirror at the predetermined position within 0.1° precision, theprecision achieved via the at least one mechanical alignment-enhancementstructure.